CARRIER SENSE MULTIPLE ACCESS (CSMA):

Transcription

1 Lecture Handout Computer Networks Lecture No. 8 CARRIER SENSE MULTIPLE ACCESS (CSMA): There is no central control management when computers transmit on Ethernet. For this purpose the Ethernet employs CSMA to coordinate transmission among multiple attached computers. CSMA is a coordination scheme that defines how to take turns using a shared cable. A computer listen to the codes i.e. it senses the carrier. If the cable is idle it starts transmitting and if the cable is in use then it waits. If simultaneous transmission occurs, the frames interfere with each other and this phenomenon is called collision. COLLISION DETECTION: As explained above, the signals from two computers will interfere with each other and the overlapping of frames is called a collision. It does not harm to the hardware but data from both frames is grabbled. ETHERNET CD: To detect the collision, Ethernet interfaces include hardware to detect transmission. It performs two operations: It monitors outgoing signals. Grabbled signal is interpreted as a collision. After collision is detected computers stop transmitting. So Ethernet uses CSMA/CD to coordinate transmission. RECOVERY FROM COLLISION: Computer that detects a collision sends special signal to force all other interfaces to detect collision. Computer then waits for other to be idle before transmission. But if both computers wait for same length of time, frames will collide again. So the standard specifies maximum delay and both computers choose random delay, which is lesser. After waiting, computers use carrier sense to avoid subsequence collision. The computer with shorter delay will go first and other computer may transmit later. 25

2 EXPONENTIAL BACK OFF: Even with random delays, collision may occur especially likely with busy segments. Computers double delay with each subsequent collision. It reduces likely hood of sequence of collision WIRELESS LANs AND CSMA/CA: IEEE is standard wireless LAN that uses radio signals at 2.4GHz. Its data rate is 11Mbps. The older devices use radio signals at 900MHz and data rate of 2Mbps. Bluetooth specifies a wireless LAN for short distances. It uses shared medium and radio waves instead of coaxial cable. LIMITED CONNECTIVITY WITH WIRELESS: In contrast with wired LANs, not all participants may be able to reach each other. Because: It has low signal strength. In wireless LANs the propagation is blocked by walls etc. It can t depend on CD to avoid interference because not all participants may hear. This is shown in the figure below: CSMA/CA: Figure 8.1 Wireless uses collision avoid ness rather than collision detection. Transmitting computer sends very short message to receiver. Receiver responds with short message reserving slot for transmitter. The response from receiver is broadcast, so all potential transmitters receive reservation. COLLISION: The receiver may receive simultaneous requests, which results in collision at receivers and both requests lost and in this way no transmitter receives reservations and both use back off and retry. The receiver may receive closely spaced requests. It selects one of them and then the selected transmitter sends message and the transmitter not selected uses back off and retries. 26

3 LOCAL TALK: Apple invented the LAN technology that uses bus topology. Its interface is included with all Macintosh computers. It has relatively low speed i.e Kbps. Also it is of low cost and we can get a free with a Macintosh, which is easy to install and connect. It uses CSMA/CA. TOKEN RING: Many LAN technologies that are ring topology use token passing for synchronized access to the ring. The ring itself is treated as a single shared communication medium. Both pass from transmitter passed by other computers and are copied by destination. Hardware must be designed to pass token ever if attached computer powered down. This is shown in figure below. Figure 8.2 USING THE TOKEN: When a computer waits to transmit it waits a token. After transmission computer transmits token on ring. Next computer is then ready to transmit, receive and then transmits. TOKEN AND SYNCHRONIZATION: Because there is only one token, only one computer will transmit at a time. Token is a short reserved frame that can not appear in data. Hardware must regenerate token if lost. Token gives computer permission to send one frame. If all computers are ready to transmit it enforces Round-Robin access. But if now computer is ready to transmit, token circulates around ring. 27

4 IBM TOKEN RING: It is very widely used. It was originally 4Mbps and now it is upto 16Mbps. It uses special connection cable between the computer and the Ring interface. FDDI: Fiber distributed data interconnect (FDDI) is another ring technology. Its most important features are: It uses fiber optics between stations and transmits data at 100Mbps. It uses pair of fibers to form two concentric rings. FDDI AND RELIABILITY: FDDI uses counter rotating rings in which data flows in opposite directions. In case of fiber a station failure, remaining stations loop back and reroute data through spare ring. In this way all stations automatically configure loop back by monitoring data ring. It is shown in figure below Figure 8.3 FDDI and Reliability: ATM ----STAR NETWORK: The ATM (Asynchronous Transferred Mode) technology consists of electronic packet switches to which the computers can connect. ATM switches form a hub into which computers can connect in a star topology. Computer gets point-to-point connections. Data from transmitters is routed directly through hub switches to destination. An ATM star network is shown in the figure below: Figure 8.4 ATM Switch 28

5 ATM DETAILS: It transmits data at over 100Mbps. It uses fiber optics to connect computer to switch. Each connection includes two fibers. It is also shown in figure. Figure